The present invention relates generally to vehicle navigation systems. More specifically, the invention relates to methods and apparatus which provide on-the-fly calibration of readings from a vehicle""s odometer sensor to ensure accurate determination of the vehicle""s position by the navigation system.
Current vehicular navigation systems are hybrids which utilize several independent position determining means to locate a vehicle. The position determining means include: global positioning system satellites (GPS), dead reckoning systems, and map databases. Typically, one among these systems will serve the primary navigation system function while the remaining determining means are utilized to recalibrate cumulative errors in the primary system. Each determining means has its advantages and limitations.
GPS is an electromagnetic wave positioning system utilized to determine a vehicle""s position. GPS includes Navstar GPS and its successors, i.e., differential GPS (DGPS), WAAS, or any other electromagnetic wave positioning system. Navstar is a GPS system which uses space-based satellite radio navigation developed by the U.S. Department of Defense. Navstar GPS receivers provide users with continuous three-dimensional position, velocity and time data. Navstar GPS consists of three major segments: space, control, and end-user segments. The space segment consists of a constellation of 24 operational satellites placed in six orbital planes above the Earth""s surface. The satellites are in circular orbits and in such an orientation as to normally provide a GPS user with a minimum of five satellites in view from any point on earth at any one time. The satellite broadcasts an RF signal, which is modulated by a precise ranging signal and a coarse acquisition code ranging signal to provide navigation data. This navigation data, which is computed and controlled by the GPS control segment for all GPS satellites, includes the satellite""s time, clock correction and ephemeris parameters, almanac and health status. The user segment is a collection of GPS receivers and their support equipment, such as antennas and processors, which allow users to receive the code and process information necessary to obtain position, velocity and timing measurements. There are two primary disadvantages to GPS positioning as it pertains to vehicular navigation. First, errors are imposed on the portion of the GPS signals accessible to civilians. The government imposes position errors in the range of 100 meters. In urban environments, this can result in inadequate navigation capabilities due to the close proximity of streets, some of which are spaced apart by less than 100 meters. The second disadvantage of GPS is that when the user is in urban environments with many scattering objects, such as buildings, it may not be possible to receive information from enough satellites to make an adequate position determination. Even where enough satellites are present, the presence of multipath errors due to the multiple reflections of the satellite signals from buildings, etc., may prevent adequate positioning on the basis of GPS alone. For this reason, GPS is typically utilized in a hybrid navigation system with other position determining means, such as dead reckoning and map databases.
Prior systems use a road network stored in a map database to calculate current vehicle positions. These systems send distance and heading information derived from either GPS or dead reckoning to perform map matching. Map matching calculates the current position based on the road network stored in the database and the input position and heading data. These systems also use map matching to calibrate sensors. The map matching, however, has inaccuracies due to map errors as well as inherent inaccuracies resulting from the fact that map matching must look back in time and match data to a location. As such, map matching can only calibrate the sensors or serve as a position determining means when an absolute position is identified on the map. However, on a long, straight stretch of highway, sensor calibration or position determination using map matching may not occur for a significant period of time.
Current land-based dead reckoning systems use vehicle speed sensors, rate gyros, reverse gear hookups, and wheel sensors to xe2x80x9cdead reckonxe2x80x9d the vehicle position from a previously known position. It is evident that the accuracy of the data received from these various sensors is essential to the reliable determination of the vehicle""s position.
The accuracy of data received from a vehicle""s odometer is influenced by a number of factors, including wheel size and pulse rate. An odometer typically detects wheel revolutions as representative of traveled distance, the tire size is directly related to the accuracy of the reported travel distance. For current navigation Systems, once the vehicle""s tire size is known, it can be manually programmed into the navigation system to properly correlate wheel revolutions to traveled distance. However, it is well known that the size of a vehicle""s tires change over time as they wear down from contact with the road. Moreover, factors such as the air pressure of the tires and the weight loaded on the vehicle at any given time produce variation in travel distances reported by the odometer. The tire size may be periodically reprogrammed into the system to account for such variations, but this is obviously impractical in that a difficult manual reprogramming would frequently be required, possibly every time the navigation system is used.
Another potential source of error in measured distance reported by an odometer is a mismatch between the odometer""s pulse rate and the pulse rate setting of the navigation system. Odometers generate a pulse train in which a specific number of pulses (e.g., 2000) represent a unit distance (e.g., a mile). For example, Nissan vehicles employ a pulse rate of 2000 pulses/mile while Ford vehicles employ a pulse rate of 8000 pulses/mile. Therefore, each navigation system must be configured to correspond to the type of vehicle in which it is installed, otherwise very large-scale errors may result. If, for example, the pulse rate setting in a navigation system installed in a Ford corresponded to the pulse rate of a Nissan, an error factor of four would be introduced. The pulse rate setting is typically done before a navigation system is installed and is difficult to change where, for example, the odometer in the vehicle is changed, or the navigation system is installed in a different vehicle. Thus, while detection of the error may be elementary, correction of the error remains problematic.
U.S. Pat. No. 5,898,390 entitled xe2x80x9cMethod and Apparatus for Calibration of a Distance Sensor in a Vehicle Navigation Systemxe2x80x9d discloses a method and apparatus for modifying an odometer reading to compensate for odometer errors resulting from pulse rate and tire size. The method and apparatus provides for correction of a first distance estimate derived from the odometer reading with a second distance estimate, typically produced by an external navigation system, i.e., GPS. Additionally, the pulse rate setting may be adjusted so as to reduce deviations between the first and second distance estimates. If pulse rate settings and tire size were the only significant sources of odometer error, the teachings of the ""390 patent would allow the production of a reliable navigation system. However, there are other far more significant sources of odometer error which the ""390 patent fails to account for, as will be discussed shortly. This failing is particularly critical in urban environments where scattering objects, such as buildings, reduce the possibility of frequent GPS initiated recalibration of the odometer based distance estimates. Absent these recalibrations, the other far more significant sources of odometer error will result in unacceptable cumulative errors in the odometer distance estimate during intervals in which GPS recalibration is not possible.
What is needed, therefore, is a method and apparatus for removing error from an odometer distance estimate in a vehicle navigation system.
The present invention enables a vehicle navigation system to automatically compensate for odometer measurement errors due to changes in tire size and/or slip, and to avoid odometer recalibration when slip is present. These capabilities improve the accuracy and reliability of the vehicle navigation system. Slip of a vehicle, e.g., a loss of traction, can occur when a road is wet or covered with snow or ice. It can occur when a road is dry and a vehicle is accelerating or decelerating rapidly. It can also occur rounding a sharp corner. Because an odometer is typically hooked up to a driven wheel, the engine or transmission, there may be large sources of error in the distance estimates derived from the odometer. Utilizing conveniently derived slip signals from an anti-lock brake system (ABS) or from a traction control system (TCS), the slip of the vehicle can be accounted for, even absent a GPS signal to recalibrate the system. This allows for improved accuracy of the system, which is particularly noticeable in urban environments where GPS signals may not be available.
In an embodiment of the invention, an apparatus for correcting odometer error in a vehicle is disclosed. The apparatus includes an odometer, a slip sensor and a first and second logic. The odometer generates an odometer indication signal indicative of a distance traveled by the vehicle. The slip sensor generates a slip indication signal indicative of a slip of the vehicle. The first logic couples to the slip sensor and the odometer to combine the slip indication signal and the odometer indication signal with a conversion parameter to form an odometer distance estimate. The second logic adjusts the odometer distance estimate with values representative of the slip indication signal to form an adjusted odometer distance estimate corresponding to the distance traveled by the vehicle during the first time interval.
In an embodiment of the invention, a vehicle navigation system is disclosed. The navigation system includes: an odometer, a slip sensor, a heading sensor, a radio navigation sensor, and a first and second logic. The odometer generates an odometer indication signal indicative of a distance traveled by the vehicle. The slip sensor generates a slip indication signal indicative of a slip of the vehicle. The heading sensor generates a heading indication signal indicative of the heading of the vehicle. The first logic converts the odometer indication signal to a first distance estimate utilizing an odometer conversion parameter. Additionally, the first logic determines a position of the vehicle based on a known prior position, the first distance estimate and the heading of the vehicle, indications of which are obtained from said heading sensor. The radio navigation sensor receives radio navigation signals. The second logic converts the radio navigation signals to an external distance estimate for the vehicle and determines whether the external distance estimate includes indicia of reliability. If the external distance estimate includes indicia of reliability, the odometer conversion parameter utilized by the first logic is adjusted.
In another embodiment of the invention, a method for determining distance traveled by a vehicle is disclosed. The method comprises the acts of:
obtaining during a first time interval an odometer indication signal and a slip indication signal;
combining a value representative of the odometer indication signal with a conversion parameter to form an odometer distance estimate; and
adjusting the odometer distance estimate with values representative of the slip indication signal to form an adjusted odometer distance estimate corresponding to the distance traveled by the vehicle during the first time interval.
In still another embodiment of the invention, a method for navigating a vehicle is disclosed. The method for navigating comprises the acts of:
obtaining during a first time interval an odometer indication signal indicative of a distance traveled by the vehicle, a slip indication signal indicative of the slip of the vehicle, and a heading indication signal indicative of the heading of the vehicle;
converting the odometer indication signal to a first distance estimate;
determining a position of the vehicle based on a known prior position, the first distance estimate obtained during said act of converting, and the heading of the vehicle, indications of which are obtained during said first act of obtaining;
obtaining during the first time interval an external distance estimate for the vehicle from a radio navigation system;
determining whether the external distance estimate includes indicia of reliability; and
adjusting an odometer conversion parameter utilized in said act of converting if the external distance estimate includes indicia of reliability as determined in said second act of determining.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.